The present invention relates to improvement of a zoom lens barrel.
Generally, in a zoom lens barrel of a mechanical compensation type, multiple groups of movable lenses, by which variable magnification and focal position adjusting operations are carried out, are optically moved relatively using multiple cam displacement units, the characteristics of the displacement amounts of which are different from each other.
Especially, in a compact camera, an exposure control member such as a shutter, and a diaphragm, are arranged between a front lens group and a rear lens group, a so-called between-shutter, and generally operations of these members are electrically controlled by a microcomputer which is connected by a flexible board provided in the camera.
On the other hand, in a single use camera or a mono-focal-point camera, generally, a mechanical shutter is used in which: a shutter is charged by a film winding operation; the charged condition is released when a shutter button is pressed; and shutter speed is controlled by the spring force.
Also in a zoom lens in which a shutter position and a diaphragm position are moved in the optical direction for short and long focal points, no problem is caused in the structure since the electric shutter is connected with the flexible board. However, since the cost of the shutter, the flexible board, and the microcomputer in the camera is high, they are appropriate for a high class camera. When a low cost camera having a zoom lens is structured, the cost of the zoom lens is too high.
Since the structure of the mechanical shutter is simple and the cost is low, it is effective for a camera such as the single use camera. However, in the zoom lens in which shutter and diaphragm positions are changed, the transmission of the operation force is difficult. When, for example, the operation force is transmitted by a shaft pin or the like, there are problems in which reliability of the shutter speed for a short focal point is varied from that for a long focal point, and the numbers of parts are increased.
Therefore, the first object of the present invention is to provide a low cost zoom lens barrel by which a highly reliable operation can be accomplished even in a zoom lens in which an exposure control member is moved in the direction of the optical axis.
Generally, in a zoom lens barrel, a lens frame is moved in the direction of the optical axis when a cam groove provided on a cam barrel or cam plate is engaged with a cam pin provided in the lens frame, Clearance is necessary to a certain extent so that the cam groove is slidably engaged smoothly with the cam pin. However, the clearance causes error or fluctuation in the direction of the optical axis of the zoom lens. In order to prevent the foregoing problems, a countermeasure is adopted in which the lens frame is pushed in the direction of the optical axis of the zoom lens, and the cam pin comes into contact with only one side wall of the cam groove. Such a countermeasure is required especially in the case where the cam member is formed by resin molding from the following reasons: a slope is provided on one side surface of the cam groove so that a metallic mold, by which the cam groove is formed, can be easily removed; and only the other surface of the cam groove is used for the cam.
FIG. 25 shows an example in which the lens frame of a two-group zoom lens, which is commonly used for a still camera, is pushed in the direction of the optical axis. In this example, the following operations are carried out: a compression spring 403 is provided between a lens frame 401 holding a front lens group G1 and a lens frame 402 holding a rear lens group G2; the lens frame 401 is pushed towards the left in the direction of the optical axis, and the lens frame 402 is pushed towards the right in the direction of the optical axis; a cam pin 404 provided to the lens frame 401 is moved in the direction of the optical axis by the left side surface of a cam groove 405a for a front lens group of a cam barrel 405 rotating around the optical axis; and a cam pin 406 provided to a lens frame 402 is moved in the direction of the optical axis by the right side surface of a cam groove 405b for a rear lens group of a cam barrel 405. Numeral 407 is a fixed barrel, numerals 407a and 407b are guide grooves provided to the fixed barrel 407 by which lens frames 401 and 402 are respectively guided-in the direction of the optical axis.
FIG. 26 shows an example in which a lens frame commonly used for a video camera is pushed in the direction of the optical axis. In this example, a lens frame 410 is pushed towards the left by a coil spring 411 provided between the lens frame 410 and the fixed barrel 407 under the condition that the lens frame 410 of a variable magnification lens group G3 which is guided in the direction of the optical axis by guide bars 408 and 409 supported by the fixed barrel 407 in parallel with the optical shaft, is penetrated by the guide bar 408. Accordingly, a cam pin, not shown in the drawing, provided in the lens frame 410 is moved in the direction of the optical axis by the left side surface of the cam groove of the cam member, not shown in the drawing.
In the conventional lens frame pushing method in the zoom lens barrel in the direction of the optical axis, there are the following problems, in which:
A. The pushing force by a spring in the case of a long focal point is varied from that in the case of a short focal point because the position of the zoom lens in the direction of the optical axis in the case of the long focal point is changed from that in the case of the short focal point, and accordingly, the driving force for a zooming operation is changed;
B. Since it is commonly known that the ratio of the coil spring length in the case where it is not activated, to the coil spring length in the case where it is activated, is not more than three, the change of the coil spring length in the case where it is activated is limited so that the ratio is not more than this value, in the case of a long focal point and in the case of a short focal point, and especially in the example shown in FIG. 26, the amount of the movement of a zooming operation is limited;
C. There is a possibility that the optical path is obscured, especially in the example shown in FIG. 25, when the coil spring is compressed and bent, and in order to avoid the foregoing possibility, when the diameter of the coil is increased, or a guide is provided, or a conical coil spring is used, a zoom lens barrel becomes large, and cost is increased;
D. Since the directions of working points of the spring and cam to the lens frame are respectively deflected in the direction crossing at right angles with the optical axis, there are the following problems: the spring and cam give force to the lens frame and the lens frame is inclined; in the example shown in FIG. 25, the lens frame 401 is inclined clockwise, and the lens frame 402 is inclined counterclockwise; in the example shown in FIG. 26, although the application point of the cam to the lens frame 410 is not shown in the drawing, when, for example, the application point is outside the guide bar 408, the lens frame 410 is inclined clockwise; and therefore, twisting is easily caused when the lens is inclined, or the lens is moved in the direction of the optical axis.
The present invention is also proposed to solve the foregoing problems. The second object of the present invention is to provide a zoom lens barrel in which: errors and fluctuations of a zooming operation caused by engagement clearance between a cam groove and a cam pin are prevented; the driving force for the zooming operation is reduced; the amount of movement for zooming can be increased; there is no possibility that a coil spring interferes with the optical path; and the spring and cam do not give force to a lens frame.
Further, the cost of a front lens group advancing motor, a microcomputer for the camera, an optical range-finding system and the like is high in an automatic focusing adjusting mechanism. Accordingly, the mechanism is suitable for a high class compact camera, but is not suitable for a low cost zoom-camera. In an ordinary picture taking range, a fixed focus type camera can be used for taking a picture on which focusing is carried out in a relatively good condition, however, there is a problem in which a subject is out of focus at infinity, or within a range in which a strobe light is used, or in a close-up photographic range within 1 m. In order to solve the foregoing problem, there is a camera such as a mono-focus camera of a film exchange type, in which an infinity-button, or a macro-button is provided so that a lens can be moved from a focusing position for an ordinary photographic distance to a focusing position for an infinite distance or a predetermined short distance when the button is pressed. However, in a zoom lens, the position of which is changable, at moment by moment a mechanism to move the lens to a focusing position becomes complicated, and the numbers of parts are increased.
The present invention has been proposed in order to solve the foregoing problems, and the third object of the present invention is to provide a variable focusing lens barrel in which: the focusing operation can be switched in two steps from the condition in which a photographic subject at a predetermined short distance is in focus, to the condition in which the subject at a longer distance is in focus; therefore, variable magnification photographing in focus can be carried out within a range from the short distance to the long distance; the structure is simple and the number of parts is small; the cost is low; and a highly reliable operation can be carried out.